CONTROLLING A LOAD COMMUTATED CONVERTER DURING UNDERVOLTAGE

Abstract

A load commutated converter interconnects an AC power grid with an AC load and comprises a grid-side converter, a DC link and a load-side converter. A method for controlling the load commutated converter comprises: determining a gridside firing angle for the grid-side converter; determining a load-side firing angle for the load-side converter; determining a grid voltage of the AC power grid; modifying the grid-side firing angle and/or the load-side firing angle based on the grid voltage, such that when an undervoltage condition in the AC power grid occurs, the operation of the load commutated converter is adapted to a change in the grid voltage; and applying the grid-side firing angle to the grid-side converter and the load-side firing angle to the load-side converter.

Claims

1. A method for controlling a load commutated converter, which interconnects an AC power grid with an AC load, the load commutated converter comprising a grid-side converter, a DC link and a load-side converter, the method comprising: determining a grid-side firing angle (α) for the grid-side converter; determining a load-side firing angle (β) for the load-side converter; determining a grid voltage (U.sub.L) of the AC power grid; modifying the grid-side firing angle (α) and/or the load-side firing angle (β) based on the grid voltage (U.sub.L), such that when an undervoltage condition in the AC power grid occurs, the operation of the load commutated converter is adapted to a change in the grid voltage (U.sub.L); applying the grid-side firing angle (α) to the grid-side converter and the load-side firing angle (β) to the load-side converter.

2. The method of claim 1, wherein the grid-side firing angle (α) is modified such that during a change from the undervoltage condition back to a normal condition in the AC power grid, the DC link current (i.sub.DC) stays below an upper bound.

3. The method of claim 1, wherein a lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on the grid voltage (U.sub.L), and the grid-side firing angle (α) is changed to the lower bound (α.sub.lim), when it is below the lower bound (α.sub.lim).

4. The method of claim 3, wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on a difference between a DC link current (i.sub.DC) and a maximal current for the DC link; and/or wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on an inductance of the DC link; and/or wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on a switching delay, after which the next switching of the grid-side converter is possible.

5. The method of claim 1, wherein an unmodified grid-side firing angle (α.sub.old), which is modified to the grid-side firing angle (α) to be applied to the grid-side converter, is determined based on a grid-side DC link voltage (U.sub.DC,CLS) which is determined from a current reference (i.sub.dw) and/or torque reference (T.sub.w).

6. The method of claim 1, wherein the grid-side firing angle (α) and/or the load-side firing angle (β) is determined based on a difference between the DC link current (i.sub.DC) and a current reference (i.sub.dw); wherein the current reference (i.sub.dw) is modified based on the grid voltage (U.sub.L) such that a power consume of the load is adapted to the power provided by the power grid during the undervoltage condition; and/or wherein the current reference (i.sub.dw) is determined based on a reference torque (T.sub.w) for the load and the torque reference (T.sub.w) is modified based on the grid voltage (U.sub.L) such that a power consume of the load is adapted to the power provided by the power grid during the undervoltage condition.

7. The method of claim 6, wherein an upper bound for the current reference (i.sub.dw) and/or the torque reference (T.sub.w) is determined based on the grid voltage (U.sub.L), and the current reference (i.sub.dw) and/or torque reference (T.sub.w) is changed to the upper bound, when it is above the upper bound.

8. The method of claim 7, wherein the upper bound for the current reference (i.sub.dw) and/or torque reference (T.sub.w) is determined based on a lower bound of the grid-side firing angle (α).

9. The method of claim 1, wherein the load-side firing angle (β) is modified such that during the undervoltage condition, a load-side DC link voltage is adapted to a grid-side DC link voltage (U.sub.DC,CLS).

10. The method of claim 1, wherein the load-side firing angle (β) is modified based on a function of the modified grid side firing angle (α).

11. The method of claim 1, wherein an unmodified load-side firing angle (β.sub.old), which is modified to the load side firing angle (β) to be applied to the load-side converter, is determined based on a look-up table.

12. A computer program for controlling a load commutated converter, which interconnects an AC power grid with an AC load, the load commutated converter comprising a grid-side converter, a DC link and a load-side converter, the computer program when being executed by a processor, is adapted to carry out the following: determine a grid-side firing angle (α) for the grid-side converter; determine a load-side firing angle (β) for the load-side converter; determine a grid voltage (U.sub.L) of the AC power grid; modify the grid-side firing angle (α) and/or the load-side firing angle (β) based on the grid voltage (U.sub.L), such that when an undervoltage condition in the AC power grid occurs, the operation of the load commutated converter is adapted to a change in the grid voltage (U.sub.L); apply the grid-side firing angle (α) to the grid-side converter and the load-side firing angle (β) to the load-side converter.

13. A computer-readable medium, in which a computer program according to claim 12 is stored.

14. A controller adapted for controlling a load commutated converter, which interconnects an AC power grid with an AC load, the load commutated converter comprising a grid-side converter, a DC link and a load-side converter, the controller structured to: determine a grid-side firing angle (α) for the grid-side converter; determine a load-side firing angle (β) for the load-side converter; determine a grid voltage (U.sub.L) of the AC power grid; modify the grid-side firing angle (α) and/or the load-side firing angle (β) based on the grid voltage (U.sub.L), such that when an undervoltage condition in the AC power grid occurs, the operation of the load commutated converter is adapted to a change in the grid voltage (U.sub.L); apply the grid-side firing angle (α) to the grid-side converter and the load-side firing angle (β) to the load-side converter.

15. A load commutated converter, comprising: a grid-side converter for converting a grid-side AC current from an electrical power grid into a DC current (i.sub.DC); a load-side converter for converting the DC current (i.sub.DC) into a load-side AC current to be supplied to a load; a DC link interconnecting the grid-side converter and the load-side converter comprising at least one inductance; a controller according to claim 14 for controlling the grid-side converter and the load-side converter.

16. The method of claim 2, wherein a lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on the grid voltage (U.sub.L), and the grid-side firing angle (α) is changed to the lower bound (α.sub.lim), when it is below the lower bound (α.sub.lim).

17. The method of claim 16, wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on a difference between a DC link current (i.sub.DC) and a maximal current for the DC link; and/or wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on an inductance of the DC link; and/or wherein the lower bound (α.sub.lim) for the grid-side firing angle (α) is determined based on a switching delay, after which the next switching of the grid-side converter is possible.

18. The method of claim 17, wherein an unmodified grid-side firing angle (α.sub.old), which is modified to the grid-side firing angle (α) to be applied to the grid-side converter, is determined based on a grid-side DC link voltage (U.sub.DC,CLS) which is determined from a current reference (i.sub.dw) and/or torque reference (T.sub.w).

19. The method of claim 18, wherein the grid-side firing angle (α) and/or the load-side firing angle (β) is determined based on a difference between the DC link current (i.sub.DC) and a current reference (i.sub.dw); wherein the current reference (i.sub.dw) is modified based on the grid voltage (U.sub.L) such that a power consume of the load is adapted to the power provided by the power grid during the undervoltage condition; and/or wherein the current reference (i.sub.dw) is determined based on a reference torque (T.sub.w) for the load and the torque reference (T.sub.w) is modified based on the grid voltage (U.sub.L) such that a power consume of the load is adapted to the power provided by the power grid during the undervoltage condition.

20. The method of claim 19, wherein an upper bound for the current reference (i.sub.dw) and/or the torque reference (T.sub.w) is determined based on the grid voltage (U.sub.L), and the current reference (i.sub.dw) and/or torque reference (T.sub.w) is changed to the upper bound, when it is above the upper bound; wherein the upper bound for the current reference (i.sub.dw) and/or torque reference (T.sub.w) is determined based on a lower bound of the grid-side firing angle (α); wherein the load-side firing angle (β) is modified such that during the undervoltage condition, a load-side DC link voltage is adapted to a grid-side DC link voltage (U.sub.DC,CLS); wherein the load-side firing angle (β) is modified based on a function of the modified grid side firing angle (α); wherein an unmodified load-side firing angle (β.sub.old), which is modified to the load side firing angle (β) to be applied to the load-side converter, is determined based on a look-up table.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The subject-matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings.

[0054] FIG. 1 schematically shows a load commutated converter according to an embodiment of the invention.

[0055] FIG. 2 shows a flow diagram for a method for controlling a load commutated converter according to an embodiment of the invention.

[0056] FIG. 3 schematically shows a controller for controlling a load commutated converter according to an embodiment of the invention.

[0057] FIG. 4 schematically shows aspects of the controller of FIG. 3.

[0058] FIG. 5 schematically shows aspects of the controller of FIG. 3.

[0059] FIG. 6 schematically shows aspects of the controller of FIG. 3.

[0060] The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0061] FIG. 1 shows a load commutated converter 10, which comprises a grid-side converter 12, an inductive DC link 14 and a load-side converter 16. Furthermore, FIG. 1 shows an AC grid 18 and an AC load 20, for example a synchronous machine, which are interconnected by the load commutated converters 10.

[0062] The grid-side converter 12 and the load-side converter 14 typically comprise a number of 6-pulse thyristor converter bridges 22. On one side, the line-side converter 12 may be connected to the three-phase AC grid 18 by means of a transformer and/or a number of filters to mitigate grid current harmonics. On the other side, the line-side converter 12 is electrically connected to the DC link 14, which again is electrically connected to the load-side converter 16. The load-side converter 16, and thus the load commutated converters 10, is connected to the AC load 20.

[0063] The grid-side converter 12 may be referred to as rectifier, while the load-side converter 16 may be referred to as inverter. However, this naming convention ignores that the power flow may also be inverted, such that the line-side converter 12 operates as an inverter, and the load-side converter 16 as a rectifier.

[0064] The depicted topology is only one possible variant. In particular, the connections between the described elements do vary. For instance, instead of a single three-phase connection, dual three-phase or multiple three-phase (polyphase) connections may be used. The grid-side converter 12 and the machine-side converter 16 may comprise multiple 6-pulse thyristor bridges. The DC link 14 may be connected as a two-port network, or in other configurations. Also parallel configurations are possible, where each entity comprises its own grid-side converter 12, DC link 14 and load-side converter 16.

[0065] Furthermore, the connection between the load commutated converter 10 and the grid 18 may comprise a transformer, circuit breakers, isolators and/or different filters. The connection between the load commutated converter 10 and the load 20 may comprise one or more filters, transformers and/or circuit breakers. Both connections may be long cables, which may induce additional dynamics to the system.

[0066] The load commutated converter 10 comprises a controller 24, which is adapted for performing a method for controlling the converter 10 during undervoltage conditions.

[0067] FIG. 2 shows a flow diagram for such a method. Details of the method will be explained with respect to FIGS. 3 to 6.

[0068] In step S10, the controller 24 determines a grid-side firing angle α for the grid-side converter 10 based on a DC link current i.sub.DC of the DC link 14 (and possible based on further quantities).

[0069] In step S12, the controller 24 determines a load-side firing angle β for the load-side converter 16, also based on the DC link current i.sub.DC (and possible based on further quantities).

[0070] In step S14, the controller 24 determines a grid voltage magnitude U.sub.L of the AC power grid 18. For example, the grid voltage magnitude U.sub.L may be measured in the connection between the grid 18 and the converter 10.

[0071] In step S16, the controller 24 modifies the grid-side firing angle α and/or the load-side firing angle β based on the grid voltage magnitude U.sub.L, such that when an undervoltage condition in the AC power grid 18 occurs, the operation of the load commutated converter 10 is adapted to a change in the grid voltage magnitude U.sub.L.

[0072] In step S18, the controller 24 applies the grid-side firing angle α to the grid-side converter 12 and the load-side firing angle β to the load-side converter 16. Here, the controller 24 determines corresponding gate signals for the thyristors of the thyristor bridges 22.

[0073] FIG. 3 shows components of the controller 24, which comprises a speed control layer (or outer loop) 26 and a current control layer (or inner loop) 28.

[0074] The speed control layer 26 comprises a speed controller 30 (typically a PI controller), which, depending on a speed setpoint n.sub.w and a speed estimate n.sub.x, generates a DC link current reference i.sub.dw or a torque reference T.sub.ref.

[0075] Furthermore, layer 26 may comprise an anti-windup controller for compensating actuator saturation. The anti-windup controller may be in situations, where the inner control loop 28 is not able to provide torque demanded by the outer control loop 26 via the torque reference T.sub.ref.

[0076] In the case, the DC link current reference i.sub.dw is not directly generated, a torque controller 32 determines the DC link current reference i.sub.dw from the torque reference T.sub.ref.

[0077] The DC link current I.sub.DC is an approximate, yet measurable measure of the drive torque and may be measured directly in the DC link 14. In the inner current control layer, the actual DC link current i.sub.DC is compared to the reference i.sub.dw, and the grid-side firing angle α is adapted accordingly. The output of the current controller 36 (typically a PI controller) is the DC side voltage U.sub.DC,CLS of the grid-side converter 12, which is proportional to cos α.

[0078] Using an average model of the grid-side converter (12)

U.sub.DC,CLS≈k U.sub.L cos α,  (1)

[0079] where U.sub.L is the grid voltage magnitude or amplitude and k is a constant, the grid-side firing angle can be computed as

α=arcos (U.sub.DC,CLS/(k U.sub.L)).  (2)

[0080] As indicated in FIG. 3, from the voltage U.sub.DC,CLS an unmodified firing angle α.sub.old for the grid-side converter 12 is determined with a grid-side angle controller 37.

[0081] The unmodified load-side firing angle β.sub.old is given from a lookup table 34 based on the DC link current i.sub.DC. The lookup table 34 is configured to ensure reliable operation of the load-side converter 16 and close to unity power factor in the stator windings, depending on the state of the load 20.

[0082] Both grid-side and load-side switching instances of the thyristors are determined from the firing angles α, β by a modulator.

[0083] During steady state, this simplifies the control and gives the highest drive efficiency due to low reactive power consumption of the load 20, such as a synchronous machine. However, during an undervoltage condition, the machine side inductor voltage is not reduced enough to maintain the DC link current i.sub.DC and thus the drive torque.

[0084] Furthermore, the controller may comprise an excitation control loop for a synchronous machine 20. In a typical setup for control of synchronous machines, the excitation control loop is an additional control loop.

[0085] For handling undervoltage conditions, the controller 24 additionally comprises a torque limiter 38, an angle limiter 40 and an angle controller 42.

[0086] Note that the controller 24 does not need to comprise all three subcontrollers 38, 40, 42 (which may be implemented as software routines). However, undervoltage conditions may be handled more efficiently by a combination of two or all three subcontrollers 38, 40, 42.

[0087] Lower Bound for the Grid-Side Firing Angle α

[0088] With respect to FIG. 4, a method (for example implemented by a software routine) for adapting the grid-side firing angle α in case of an undervoltage condition is described. When the line side voltage returns after an undervoltage condition, there may be a risk of an overcurrent in the DC link 14. The reason is that there is a delay inherent in the switching (i.e. there is an actuator delay) which limits the speed of the reaction of the controller 24. The actuator delay may not be constant, but may depend on the firing angle α (control input). The method may deal with the delay by limiting the value of the grid-side firing angle α.

[0089] In this situation, it may be assumed that the line voltage magnitude U.sub.L has dropped to a level U.sub.L<1 and that the control system has stabilized and is applying grid-side converter and machine-side converter firing angles α.sup.0, β.sup.0, respectively, and the DC link current is i.sub.DC.sup.0. It may be furthermore assumed that at time t.sub.0 the voltage amplitude returns instantaneously to the nominal value with a step ΔU.sub.L:=1−U.sub.L. If the control of the grid-side converter firing angle has a delay of T.sub.d, the DC link current will grow according to

d/dt Δi.sub.DC=ΔU.sub.L cos(α.sup.0)/L.sub.DC, t in [t.sub.0, t.sub.0+T.sub.d],  (3)

[0090] where Δi.sub.DC is the deviation from the steady state i.sub.DC.sup.0. Thus, when the controller 24 can react (at time t.sub.0+T.sub.d), the DC link current is at the value i.sub.DC=i.sub.DC.sup.0+T.sub.d ΔU.sub.L cos (α.sup.0)/L.sub.DC. In order not to violate the current limit i.sub.DC,lim, we impose therefore the limit

cos (α.sup.0)≦(i.sub.DC,lim−i.sub.DC.sup.0) L.sub.DC/(ΔU.sub.L T.sub.d)

α.sup.0≧arcos ((i.sub.DC,lim−i.sub.DC.sup.0) L.sub.DC/(ΔU.sub.L T.sub.d)).  (4)

[0091] As mentioned above, the value of the delay T.sub.d is time varying. In the proposed embodiment we consider the worst case which is a switching delay of 60°. With a line frequency of 50 Hz, this corresponds to 0.02*1/6≈3 ms.

[0092] Note that the presented limit on the grid-side converter firing angle α is conservative. Other bounds on, or adaptations of the grid-side firing angle α based on the grid voltage U.sub.L are possible.

[0093] The undervoltage adaptation of the grid-side firing angle α can thus be stated as

α.sub.lim=arcos ((i.sub.DC,lim−i.sub.DC) L.sub.DC/ΔU.sub.L T.sub.d)),  (5)

α=max(α.sub.old, α.sub.lim)  (6)

[0094] with α.sub.lim being the limit on the firing angle stemming from the observation above, α.sub.old being the unmodified firing angle, and α being the adapted firing angle.

[0095] As indicated in FIG. 4, which shows components of the subcontroller 40, in block 34, the limit on the firing angle α.sub.lim is computed from the grid voltage magnitude U.sub.L and the DC link current i.sub.DC using Equation (5). In block 46, this limit is compared to the unmodified grid-side converter firing angle α.sub.old selected by the controllers 36, 37, and is possibly adapted to the new grid-side firing angle α.

[0096] Adaptation of the Torque Reference T.sub.ref

[0097] When the line voltage drops, we adapt the lower limit of the grid-side firing angle α as described above. When the firing angle α is restricted, this also restricts the power which can be taken from the grid 18 and thus the maximum torque which a load 20 can deliver. Assuming an ideal voltage source, constant power outtake can be kept during an undervoltage condition by increasing the current. However, the DC link current i.sub.DC cannot be arbitrarily large. Therefore, the torque reference T.sub.ref may be adjusted whenever the firing angle α is adjusted. Adjusting the torque reference T.sub.ref may help to avoid windup in the controller, and the occurrence of limit cycles with oscillating torques being delivered to a synchronous machine.

[0098] The adjustment of the torque may be based on a power balance consideration. The power on the grid side and on the load side must be equal,

k U.sub.L cos(α) i.sub.DC=ω.sub.r T.sub.ref,  (7)

[0099] where k is a constant, T.sub.ref is the torque reference and ω.sub.r is the rotor speed. If the firing angle limit is such that kU.sub.Lcos(α.sub.min)i.sub.DC,lim≧ω.sub.rT.sub.ref (where α.sub.min is the lower bound on the grid-side converter firing angle and i.sub.DC,lim is the upper limit on the current), we lower the torque reference in order to be able to satisfy the power balance with a DC current satisfying the upper limit. We apply the following algorithm to the torque reference,

T.sub.ref,lim=(k U.sub.L cos(α.sub.min) i.sub.DC,lim) /ω.sub.r.  (8)

T.sub.ref=min(T.sub.ref,old, T.sub.ref,lim).  (9)

[0100] Here, the limit on the torque reference is denoted by T.sub.ref,lim, the unmodified torque reference is denoted by T.sub.ref,old and the modified torque reference is denoted as T.sub.ref.

[0101] As indicated in FIG. 5, which shows components of the subcontroller 40, in block 48, an upper bound T.sub.ref,lim on the torque reference is computed from the grid voltage magnitude U.sub.L, the upper limit on the DC link current i.sub.DC, the lower limit on the grid-side converter firing angle α.sub.lim and the rotor speed ω.sub.r, using Equation (8). This bound is then compared to the torque reference T.sub.ref,old from the speed PI controller in block 50 and the smaller value is taken as modified torque reference T.sub.ref.

[0102] Analogously, the DC link current reference i.sub.dw or equivalent quantities may be modified.

[0103] Adaptation of the load-side firing angle β

[0104] Finally, a third method is described with respect to FIG. 6, which adapts the load-side firing angle β in case the grid-side firing angle α saturates.

[0105] Instead of choosing a constant value β.sub.old for the load-side firing angle from a lookup table 34, feedback control may be implemented. For instance, the load-side firing angle β may be controlled to be decreased as a function of the grid voltage U.sub.L, or as a function of deviation between current reference i.sub.dw and actual current i.sub.DC.

[0106] With reference to FIG. 1, a DC link voltage U.sub.DC is applied over the DC link inductance. This DC link voltage U.sub.DC is the difference of the DC side voltage of the grid-side converter U.sub.DC,CLS and the DC side voltage of the load-side converter U.sub.DC,CMS.

[0107] When the current PI controller demands a DC side voltage U.sub.DC,CLS,ref higher than what is feasible—be it due to an undervoltage condition or due to the subsequently imposed lower bound on the grid-side converter firing angle α—the method may reduce the DC side voltage of the load-side converter U.sub.DC,CMS accordingly, to keep the voltage applied over the DC link inductance unaffected by the saturation. Thus, the PI controller continues to control the DC link current i.sub.DC, yet by means of the load-side converter 16. Reducing the DC side voltage U.sub.DC,CMS of the load-side converter 16 may reduce the power supplied to a load 20. However, this reduction of the consumed power ensures that the DC link current i.sub.DC does not vanish as would happen if the DC side voltage of the machine-side converter U.sub.DC,CMS is kept constant. Since the DC link current i.sub.DC may be essential to generate drive torque, the method may ensure that the drive is able to deliver at least some of the requested torque during undervoltage conditions.

[0108] The adapted DC side voltage of the machine-side converter U.sub.DC,CMS,new can be computed from the difference between the requested and the actual (saturated) voltage on the grid-side converter side,

U.sub.DC,CMS,new=U.sub.DC,CMS,old−(U.sub.DC,CLS,ref−U.sub.DC,CLS,act).  (10)

[0109] Inserting the average models for the voltages of the grid-side converter 12 and the load-side converter 16,

U.sub.DC,CLS≈k U.sub.L cos α, U.sub.DC,CMS≈−k U.sub.M cos β  (11)

[0110] yields the update formula for the load-side converter firing angle

β=arcos (cos β.sub.old+U.sub.L cos α.sub.old/U.sub.M−U.sub.L cos α/U.sub.M).  (12)

[0111] The unmodified load-side firing angle from the lookup table 34 is denoted by β.sub.old, and the modified firing angle is denoted by β. As above, we also have the grid-side firing angle α.sub.old from the current PI controller, and the modified grid-side firing angle α.

[0112] As indicated in FIG. 5, which shows components of the subcontroller 40, in block 52, the adapted load-side firing angle β is computed from the grid voltage magnitude U.sub.L, the machine (load) voltage magnitude U.sub.M, the grid-side converter angle α.sub.old requested by the current PI controller, the modified grid-side firing angle α and the modified load-side firing angle β.sub.old from the lookup table 34.

[0113] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practising the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

LIST OF REFERENCE SYMBOLS

[0114] 10 load commutated converter [0115] 12 grid-side converter [0116] 14 DC link [0117] 16 load-side converter [0118] 18 power grid [0119] 20 load [0120] 22 thyristor converter bridge [0121] 24 controller [0122] α grid-side firing angle [0123] β load-side firing angle [0124] i.sub.DC DC link current [0125] U.sub.L grid voltage magnitude [0126] U.sub.DC DC link voltage [0127] U.sub.DC,CLS DC side voltage of the grid-side converter [0128] U.sub.DC,CMS DC side voltage of the load-side converter [0129] 26 speed control layer [0130] 28 current control layer [0131] 30 speed controller [0132] 32 torque controller [0133] 34 lookup table [0134] 36 current controller [0135] 37 grid-side angle controller [0136] 38 torque limiter [0137] 40 grid side angle limiter [0138] 42 load-side angle controller [0139] α.sub.old unmodified grid-side firing angle [0140] β.sub.old unmodified load-side firing angle [0141] n.sub.w speed setpoint [0142] n.sub.x speed estimate [0143] T.sub.ref torque reference [0144] i.sub.dw DC link current reference [0145] α.sub.lim lower bound for grid-side firing angle [0146] T.sub.ref,old unmodified torque reference [0147] T.sub.ref,lim upper bound for torque reference [0148] 44 controller component [0149] 46 controller component [0150] 48 controller component [0151] 50 controller component [0152] 52 controller component [0153] U.sub.M machine voltage magnitude